Is my description of how a gyro-compass works correct?

I need to explain – in laymen’s language – the workings of a typical WWII-era Sperry Mark XIV gyro-compass. These were built by the thousands by Chrysler (the auto manufacturer) and installed on U.S. warships in the 1940s.

I did a bunch of online research and this is how I think it works. Now granted, I have glossed over some of the more complicated mechanisms and principles, but that’s okay – remember, this is aimed at laymen.

So, is this essentially correct? If not, what did I get wrong? Thanks all, in advance.

I don’t think they actually detect true north at all. I think they just keep pointing in the same direction they start off pointing, and that has to be re-calibrated occasionally.

Chronos, I think you are mistaken. I believe heading finders, the first cousins of gyro-compasses which are used primarily in aircraft, work the way you describe. Shipboard gyro-compasses find true north.

I don’t think your description is quite accurate. Maybe something more like this:

At the heart of the gyro-compass is a very heavy spinning bronze wheel called a rotor which is mounted on a gimble (a 3-dimensional pivot). A spinning rotor will tend to stay in a fixed orientation. It will resist movements that try to turn it. This can be demonstrated by a bicycle wheel that you hold with outstretched hands. If the wheel is not spinning, it will move wherever you point it. But once the wheel is set spinning, it will resist your attempts to reposition it.

As the earth rotates, the spinning rotor of the gyro-compass is carried along with it. But since the rotor is is spinning, it orientation remains fixed in absolute space. If you watched the rotor for 24 hours, you’d see its orientation slowly trace out a full circle. Of course, its orientation isn’t really changing – your orientation is changing as the earth rotates underneath you.

In order to get the rotor to point north, a mercury balance system is used. The balance system is a set of small chambers connected with tubes and filled with mercury. It’s attached to the rotor mount. As the rotor changes its orientation in response to the earth’s rotation, the balance system tilts, and gravity causes the mercury to flow through the tubes from one chamber to another. This slightly unbalances the rotor assembly, applying a small force. The balance system is designed so this force gradually nudges the spinning rotor back toward north.

I am just an engineer so I could be wrong but I believe they do find true north. If a ship is in port for a long period of time the gyro is shut off and the wheel stops spinning. I can remember some discussions of spinning up the gyro before leaving port.

What is not clear to me from this is why the mercury causes just enough change but not too much to align to North. Why is the force just the right amount?

I’m not sure it’s possible to explain this without getting really technical. Basically, when you apply a force to a spinning gyroscope it doesn’t move in the direction you expect it to, it rotates off to the side. The mercury system is designed so that tilt introduces a force that causes a sideways rotation of the rotor back toward north and re-levels the whole thing. The stable point of the system is where the rotor is horizontal and pointing north (or south) – any other state is unstable.

Two comments:

  1. I prefer Hamster King’s description over the OP’s. The gyro is the primary component & the North-seeking mercury rig is secondary. Once you have the gyro spun up you could manually adjust it to match to a magnetic compass if the mercury north-sensor was inoperative. But without the gyro, you got nuthin’.

Gyro compasses in small aircraft are usually pure gyros; they have no idea where North is; they are simple 1D directionally-stabilized platforms.

Operationally they are manually set to the correct magnetic heading after start-up by reference to a conventional magnetic compass & are periodically manually reset to the same magnetic compass to eliminate accumulated drift. A drift rate of 5 degrees / hour would not be unusual.

Aircraft compasses on more complex aircraft dating from the 1940s forward to 10-ish years ago usually include one or more “flux gates” ( which are sensors which use a circulating electrical current to detect the direction of magnetic North. These automatically & continuously nudge the gyro to obtain & maintain alignment with magnetic North. There’s also a manual setting to disable the flux gate input or to rapidly slew the gyro into rough alignment with a separate magnetic compass & then rely on the flux gate to fine tune & maintain alignment to magnetic North.

The latest & greatest in aircraft compass systems replace the traditional flux gate with more modern solid state magnetometers but otherwise operate on the same principals.

So how does it know when it is pointing north or south? Is mercury sensitive to a magnetic field?

It wants to point perpendicular to both the direction of Earth’s spin and the direction of gravity, which happens to be north/south.

Ah…so it aligns to absolute N/S, not magnetic.

OK, I’ve learned something new. I would not have thought it possible to detect the Earth’s rotation with such a small instrument. Huh.

So what’s the point of having a gyro if you’re just going to continually force it to agree with the magnetic compass? Why not just have the magnetic compass, and skip the middleman?

Magnetic compasses have all sorts of nasty behaviors when placed in a vibrating environment moving & accelerating in all 3 dimensions. As long as you’re going straight & level and not accelerating and not near a Pole they work OK. But the instant you roll into a bank, climb, descend, speed up or slow down, the compass swings way off the correct reading.

Which makes turning from one heading to another rather hard if the instrument you use to know when to stop turning is lying to you. Conversely, a raw gyrocompass has none of those issues and is reliable and repeatable for many minutes regardless of the aircraft’s motion. But it doesn’t know where North is.

Combine the two & you get the best of both worlds, at the cost of needing to add some system to periodically sync the gyro to the magnetic compass. Back in the early days of aviation that “system” was the pilot. It remains that way on small airplanes today for reasons of cost.

This 3MB PDF from the FAA talks about all kinds of aircraft instruments. Pages 10-12 cover compass design & pages 13-15 cover compass errors as well as self-aligning flux gates.